This video aims to show:

1. For screw gear drive with parallel shafts, i.e. spur gear drive of helical teeth (in green): helix angles of the two gears must be equal and of opposite hands.

2. For screw gear drive with skew perpendicular shafts (in yellow and in pink): helix angles of the two gears B1, B2 must be of same hand, B1 + B2 = 90 deg. The yellow drive is of right hand. The pink drive is of left hand. If input gears (small ones) rotate in the same direction, the output gears (large ones) rotate in opposite directions.